Carbon nanotubes (CNTs) are carbon allotropes having a generally cylindrical nanostructure. They have unusual properties that make them valuable for many different technologies. For instance, some CNTs can have high thermal and electrical conductivity, making them suitable for replacing metal heating elements. Due to their much lighter mass, substituting CNTs for metal heating components can reduce the overall weight of a heating component significantly. This makes the use of CNTs of particular interest for applications where weight is critical, such as in aerospace and aviation technologies.
Carbon nanotubes are commercially available in several different forms; however, these off-the-shelf CNTs are not suitable for all aircraft ice protection applications. One such form is as a CNT yarn. Carbon nanotube yarn includes aligned bundles of CNTs that are hundreds of microns in diameter and millimeters long. In a CNT yarn, carbon nanotubes are spun into long fibers and can be plied together or stretched to provide the desired length and mechanical properties. However, while CNT yarns have been used in heating elements, they have first been incorporated into a substrate film. Incorporating the CNT yarn into a film reduces some of the flexibility that CNTs provide. For example, CNT films typically contain air voids that increase the electrical resistivity (and reduce the conductivity) of the CNT film. As a result, CNT yarns incorporated into a film may not provide the level of electrical resistivity necessary for many aerospace heating applications (e.g., anti-icing and de-icing). Thus, forming a film with a commercially available CNT yarn cannot currently be used as a substitute for metal heating elements.
Other forms include pure carbon nanotube nonwoven sheet material (CNT-NSM) and CNT-filled thermoplastic films. In a CNT-NSM, carbon nanotubes are arranged together to form a sheet. No adhesives or polymers are typically used to attach CNTs to one another in a CNT-NSM. Instead, CNT particles are attached to one another via Van der Waals forces. In a CNT-filled thermoplastic film, individual CNT particles are distributed throughout the film. Unfortunately, these commercially available CNT materials do not offer off-the-shelf electrical resistivities that allow for their use in different ice protection applications. For example, the electrical resistivity of commercially available, CNT-filled thermoplastic films is generally in the range of 3×10−4 ohms-cm (Ω-cm) or higher and the electrical conductivity of commercially available, off-the-shelf CNT-NSMs is generally in the range of 350-400 S/cm or lower. These levels of electrical resistivity are not suitable for many aerospace heating applications. Thus, commercially available CNT-filled thermoplastic films and CNT-NSMs cannot currently be used as a substitute for metal heating elements.